Fall back to ether_ioctl() by default.

The common pratice in ethernet device drivers is to fall back to
ether_ioctl() to implement generic ioctls not implemented by the driver
and to fail if no handler exists.

Convert these drivers to follow that practice rather than calling
ether_ioctl() for specific cases.

vxge(4) aready had the default case, but it was only called on failure
to match.

Reviewed by: imp
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14895